The invention relates to arrangements for discharging exhaust gases so as to decrease the amount of pollutants therein. More particularly, the invention also relates to pollution lessening arrangements which include provision for providing additional air to more fully oxidize and reduce pollutants in the engine exhaust gases.
It has been known in the automotive art to provide an air pump to be driven off the drive shaft of an internal combustion engine to supply air into the exhaust stream upstream of a catalytic converter. These catalytic converters were found in early automotive attempts to install catalysts and comprised oxidizing catalysts that would burn or oxidize the fuel not oxidized in the combustion chamber. These engines did not have to be concerned about reducing NOx due to the regulations at the time. Accordingly, a relatively large air pump was included and additional air was pumped into the exhaust system upstream of an oxidizing catalyst. Little or no attempt was made to provide a stoichiometric environment, immediately in front of or in the catalyst.
At the present time, exhaust emissions regulations require engine designers to be concerned about three major pollutants, unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). The amount of HC and CO can be decreased by oxidizing these constituents in a catalytic (or thermal) reactor to form water and carbon dioxide. With a catalyst, the NOx can also be decomposed to form oxygen and nitrogen, but this reaction normally requires a reducing atmosphere, an absence of oxygen, to take place. With a proper catalyst and control of gases entering the catalyst, it is possible to produce a three-way catalyst; one which will oxidize the CO and HC and simultaneously reduce the NOx. However, to optimize these reactions it is necessary to hold the air-fuel ratio of the reactive constituents within a narrow range around a stoichiometric air-fuel ratio as shown in FIG. 2.
The three-way catalyst is currently a fairly common technique for reducing the emissions of automobile engines. Typically, stoichiometry is assured by using an oxygen sensor to determine the state of the exhaust mixture after discharge from the engine and before the catalyst. The signal from the sensor is fed back to an electromic controller which then regulates the amount of fuel delivered to the charge forming means, typically a fuel injection system, to,create a stoichiometric air/fuel ratio in the charge forming means upstream of the combustion chambers. No extra air is added at 0r before the catalyst. From the standpoint of the engine, the stoichiometric air/fuel mix produces very good fuel efficiency and low overall emissions, but is not necessarily optimum for power or running quality and, because of high internal temperatures, it can adversely effect engine life, especially exhaust valve life when running at high engine loads.
Attention is directed to the following U.S. patents:
______________________________________ 2,956,865 D. P. Williams October 18, 1960 3,058,299 W. M. Dean October 16, 1962 3,059,419 J. W. Schnabel October 23, 1962 3,082,597 R. J. J. Hamblin March 26, 1963 3,091,078 T. A. Dworak May 28, 1963 3,116,596 Boehme, et al. January 7, 1964 3,396,535 L. W. Milos August 13, 1968 3,672,172 G. L. Hammond June 27, 1972 3,771,921 Rohde, et al. November 13, 1973 3,852,959 T. W. Weisgerber December 10, 1974 3,862,540 B. J. Harvey January 28, 1975 4,098,078 P. A. Laurent July 4, 1978 4,276,745 Takada, et al. July 7, 1981 5,167,934 Wolf, et al. December 1, 1992 ______________________________________
U.S. Pat. No. 5,167,934 discloses a marine propulsion system with a catalyst having a reducing part, and an oxidizing part located axially downstream of the reducing part, with secondary air of undefined quantity introduced to the oxidizing part only.
Attention is also directed to a paper entitled "An Historical Overview of Emission-Control Techniques for Spark-Ignition Engines: Part B--Using Catalytic Converters" written by J. R. Mondt of General Motors Corporation which is undated.